In general the answer will depend on the phone manufacturer.
The specs tell manufacturers what they must do to claim
compliance, and many do more than the bare minimum.  It's
really up to them.

I use STK to read and log NMR from Siemens phones.  I'm able to
get much the same data (and more) using specific commands on a
Wavecom modem, and I find no real discrepancy between my
Siemens and Wavecom data (ignoring RXQUAL, which is hopeless in
idle mode).  Signal strengths (serving cell and 6 strongest
neighbours) will vary a bit, but that's to be expected.
Reassuringly, I get good agreement on the BSIC and BCCH of the
neighbouring cells, even in dynamic environments.  And most
importantly, BSIC and BCCH always pair with each other exactly.

GSM 04.08, as mentioned in GSM 11.14.  And that raises an
interesting issue.  One of the parameters buried within NMR is
the MEAS-VALID bit which "indicates if the measurement results
for the dedicated channel are valid or not" (GSM 04.08, Table
10.5.47).  A strict interpretation of GSM 11.14 indicates that
we shouldn't rely on this bit's value, but my feeling is that
it's probably OK.  It certainly seems to be dependable on
Siemens phones.

As to how often they're updated, my Siemens experience suggests
at least every 10 seconds.

John